Formation pressure measurements made from wells play an important role in the management of reservoirs of underground fluids such as oil and gas. Because of their dynamic nature formation pressure measurements provide essential information on well productivity and dynamic reservoir description both in exploration and exploitation scenarios. Static pressure data can be used to compute formation fluid density and contacts. This can be important to determine reserves. Pressure transient data on the other hand can be important for estimating permeability and heterogeneity and average reservoir pressure.
Traditionally, pressure transient testing has taken the form of Drill Stem Testing (DST) or conventional well testing in which a well is put under test for a relatively long duration. While these can be excellent ways to meet test objectives, environmental and cost considerations do not allow use these techniques at all times. Wireline and LWD tools have been developed to make probe-based formation pressure measurements to address this issue.
Wireline and while-drilling formation testers counter many of the restrictions imposed by conventional well tests. While the theory of pressure transient analysis is applicable to data obtained by such formation tests, they require formulation to account for additional effects. Specifically, formation testers can be used during measurement while drilling. However, interpretation of the pressure data acquired in this dynamic environment can be challenging. One of the difficulties arises due to supercharging which results from mud filtrate invasion and changes significantly over the duration of drilling. This results in an increase in sandface pressure which is over and above the reservoir pressure. Therefore, any calculation of initial pressure and permeability must take into account the supercharging effect.
While drilling, the well bore pressure is normally maintained at a pressure substantially greater than the formation pressure by the use of drilling fluids to control production of formation fluids into the well bore (the drilling fluids or ‘muds’ are pumped through the wellbore and are also used for cuttings transport, cleaning of the drill bit and chemical stabilisation of the well). When a producing zone is penetrated, the wellbore sandface (the region of the wellbore wall in the producing zone) is exposed to mud pressure and filtrate immediately invades the near wellbore region. A mud cake is formed when drilling fluid flows into the formation and solids are deposited at the surface of the wellbore. This process is referred to as static filtration. As the mud cake grows it eventually stabilizes to a maximum thickness. This is as a result of the shearing action of the mud circulation as well as the mechanical action of the rotating drill pipe. This process is known as dynamic filtration. During these processes a pressure gradient is established in the formation.
A schematic description of the pressure profile with supercharging effect is shown in FIG. 1. The pressure in the wellbore near the surface of the mud cake is at hydrostatic pressure (pm) but drops rapidly across the mud cake (pa) and then gradually reduces in the formation, approaching formation (farfield) pressure (pI) some distance away from the wellbore. This near wellbore elevation in pressure above the farfield is known as the supercharging effect. From the above it is clear that if a pressure transient measurement were taken soon after drilling, any interpretation technique would have to take into account the effect of supercharged pressure.
Various techniques have been proposed to address the supercharging effect. Examples can be found in U.S. Pat. No. 5,602,334, PROETT, Mark, et al. FORMATION TESTING IN THE DYNAMIC DRILLING ENVIRONMENT. SPWLA 45th Annual Logging Symposium. June 6-92004., PROETT, Mark, et al. Formation Testing In the Dynamic Drilling Environment. IADC/SPE Drilling Conference. 2-4 Mar. 2004., PROETT, Mark, et al. Supercharge Pressure Compensation Using a New Wireline Testing Method and Newly Developed Early Time Spherical Flow Model. SPE 36524. 6-9 Oct. 1996, p. 329-342., GOODE, Peter, et al. Analytical models for a multiple probe formation tester. SPE 20737 September 1990., GOODE, Peter, et al. Influence of an invaded zone on a multiprobe formation tester. SPE Formation Evaluation. March 1996, p. 31-40.
This invention aims to provide a method of interpreting formation measurements that can account for the effect of supercharging.